I. Field of the Invention
The present invention relates to the field of Quantum Electronics, and more particularly to the element basis of laser technology, and can be used for developing tunable solid state lasers.
Primarily, the invention can be used in cases when monochromatic laser emission tunable in the visible-infrared spectral region is required for solving problems in various fields of science and technology, such as laser spectroscopy, photo chemistry, photo biology, medicine, and the like.
II. Description of the Prior Art
F.sub.2.sup.+ color centers (CCs) in alkali-halide crystals constitute a pair of neighboring anion vacancies (located along the [110] axis), with one captured electron. The fact that the anion vacancies are equivalent was used to compare the energy levels of the F.sub.2.sup.+ centers with those of the hydrogen molecular ion, H.sub.2.sup.+. Such an approach, though not very accurate, provides a satisfactory consistency for the energies of transitions. At a distance R between the anion vacancies in a medium with dielectric constant E, the values of the F.sub.2 +transition energies can be obtained from the equation ##EQU1## where the values .epsilon. and R are fitted to attain better agreement with experimental results.
The F.sub.2.sup.+ CC energy level diagram is presented in M. A. Aegerter and F. Luty, The F.sub.2.sup.+ center in KCL crystal. Part 1. Formation and bleaching kinetics, Phys. Stat. Sol. (b), 43, 227-243 (1971), which is herein incorporated by reference.
The levels are marked analogously with those of the H.sub.2.sup.+ molecular ion. The low-lying optical transition is the transition 1s.sigma..sub.g .fwdarw.2p.sigma..sub.u. The two higher-energy transitions, 1s.sigma..sub.g .fwdarw.2p.pi..sub.u, are in the region of the F absorption band. It should be noted that the quantum yield of the radiational transition from the upper 2p.sigma..sub.u level, presented in the diagram, decreases practically to zero with increasing temperature up to 100 K. At temperatures above 100 K, only emission from the lower excited 2p.sigma..sub.u level can be observed. The quantum yield of this F.sub.2.sup.+ emission, which is observed for all alkali-halide crystals is rather high. For instance, according to I. A. Parfianovich, V. M. Hulugurov, B. D. Lobanov, and N. T. Maximova, Luminescence and stimulated emission of color centers in LiF, Bull. Acad. Sci. USSR, Phys. Ser., 43, 20-27 (1979), which is incorporated herein by reference, for LiF:F.sub.2.sup.+ .eta.=0.52 at T=90 K, and .eta.=0.15 at T=300 K.
Some spectroscopic data for pure F.sub.2.sup.+ CCs in LiF are presented below. This medium is one of the most promising among CC crystals for producing laser operation at room temperature. The first most comprehensive studies of the F.sub.2.sup.+ CCs optical properties were performed by J. Nahum, Optical properties and mechanism of formation of some F-aggregate centers in LiF, Phys. Rev., 158, 814-825 (1967), which is incorporated herein by reference. A relative oscillator strength of the optical transition 1s.sigma..sub.g .fwdarw.2p.sigma..sub.u at .lambda..sub.max =645 nm was determined experimentally to be: f.sub.F.sbsb.2 /f.sub.F2 +=1.1. The temperature dependence of the absorption band half-width was determined by the excitation spectrum to correspond satisfactory to formula (2) ##EQU2## with parameters .DELTA..upsilon.(0)=2450 cm.sup.-1, .omega..sub.o =2.pi..multidot.6.8.multidot.10.sup.12 s.sup.-1, S.sub.o =21.1, where .DELTA..upsilon.(0) is the multiphonon band half-width at O K, .omega..sub.o is the effective phonon frequency, and S.sub.o is the effective Huang-Rhys factor.
The luminescence band with .lambda..sub.max =910 nm and half-width 1730 cm.sup.-1 corresponds to the radiational transition 2p.sigma..sub.u .fwdarw.1s.sigma..sub.g of the F.sub.2.sup.+ CCs in LiF at 77 K. The temperature dependence of the luminescence band half-width is determined by formula (2) with parameters .DELTA..upsilon.(0)=1720 cm.sup.-1, .omega..sub.o =2.pi..multidot.7.4.multidot.10.sup.12 s.sup.-1, S.sub.o =8.74. The F.sub.2.sup.+ luminescence band shape in LiF is well described by the Gaussian curve. The measured lifetime of the relaxed excited at 77 K state equals 29 ns.
The ionic configurations of the F.sub.2.sup.+ and F.sub.A (II) CCs are identical in a relaxed excited state. This enabled F. Luty, who studied their properties comprehensively, to suggest to the Bell Telephone group in 1974, their use as a laser active center. W. Gellermann, K. P. Koch and F. Luty, Recent progress in color center lasers, Laser Focus, 71-75 (April 1982), incorporated herein by reference. Lasing on F.sub.2.sup.+ CCs at liquid nitrogen temperature was done for the first time by Mollenauer. L. F. Mollenauer, Dye like lasers for the 0.9-2 .mu.m region using F.sub.2.sup.+ centers in alkali-halides, Opt. Lett., 1, 164-154 (1977); L. F. Mollenauer, D. M. Bloom and A. M. DelGaudio, Broadly tunable cw lasers using F.sub.2.sup.+ for the 1.26-1.48 and 0.82-1.07 .mu.m bands, Opt. Lett., 3, 48-50 (1978), incorporated herein by reference. Later on, it was this medium that enabled state-of-the-art efficiencies (60%) and average power of continuous lasing (up to 1.8 W). However, the problem of fast degradation of the laser output power has prevented the production of the laser commercially. Therefore, it has been available only for highly specialized experts.
Room temperature lasing of F.sub.2.sup.+ CCs in LiF crystal was attained for the first time by Gusev et al. under ruby laser pumping. Yu. L. Gusev, S. I. Marennikov, and V. P. Chebotaev, Lasing effect in the spectrum region of 0.88-1.2 .mu.m using F.sub.2.sup.+ and F.sub.2.sup.- color centers in LiF, Sov. Tech. Phys. Lett., 3, 124 (1977), incorporated herein by reference. The efficiency of the laser was not high (2-6%), and the thermal degradation of F.sub.2.sup.+ CCs at 300 K with a half decay time of 12 h prevented the crystal from being used for more than 24 h.
A group at Irkutsk State University, Russia doped the original crystals with impurities to stabilize F.sub.2.sup.+ CCs. The LiF crystals doped with hydroxyl ions enabled the formation of F.sub.2.sup.+ (OH) CCs that are thermostable up to a temperature of 380 K. However, besides the thermostabilizing effect of the hydroxyl ions, some decline of the laser output characteristics was observed.
LiF:F.sub.2.sup.+ crystals have significant advantages as efficient active media for lasers tunable in the 820-1150 nm spectral region. The high quantum efficiency to temperatures above room temperature, the considerable values of absorption and emission cross-sections, and the frequency tuning region exceeding those for the majority of known laser media have roused great interest in this medium. Unfortunately, low thermal stability of F.sub.2.sup.+ centers (half decay time at 300 K is about 12 hours) previously prevented wide use of LiF:F.sub.2.sup.+ lasers.
To solve the problem of LiF:F.sub.2.sup.+ active media stability a new technique for two-step photoionization of neutral F.sub.2 centers in LiF was proposed. Utilization of the proposed method of F.sub.2 .fwdarw.F.sub.2.sup.+ transformation by the focused pump radiation resulted in an efficient (up to 40%) lasing of unperturbed F.sub.2.sup.+ centers. However, the problem of the active element operational stability has not been solved completely. Each active zone of the crystal pumped by the focused radiation of 532 nm experienced exhausting due to thermal degradation of F.sub.2.sup.+ centers. Thus crystals could be used for 10.sup.2 -10.sup.3 multi-hour cycles before they have to be replaced.
The thermal stability of positively charged F.sub.2.sup.+ centers can be raised by applying suitable anion or cation impurity doping during crystal growth. In this case new F.sub.2.sup.+ like optical centers are formed, which are perturbed by dopant ions or by components of dopant destruction. For example: the F.sub.2.sup.+ *-F.sub.2.sup.+ center, perturbed by Me.sup.++ ions in LiF & NaF crystals; the F.sub.2.sup.+ **-F.sub.2.sup.+ center, perturbed by O.sup.- ions in LiF, NaF, NaCl, KCl, KBr crystals; and the (F.sub.2.sup.+).sub.A -F.sub.2.sup.+ center, perturbed by Li in NaF and KCl crystals.
However, this idea for room temperature thermostabilization of F.sub.2.sup.+ like centers does not always provide appropriate photostability of active medium under powerful laser excitation. For example, pumping of the LiF:F.sub.2.sup.+ ** stabilized crystals by the radiation of the second harmonic of YAG:Nd laser results in a significant fading of the color center laser output. This fading is caused by the complex photo-chemical process in the pumping channel of the crystal, which involves two-step photoionization of neutral F.sub.2 centers and trapping of the released electron by the positively charged F.sub.2.sup.+ centers.
In the proposed invention the problems related to photo and thermostability of LiF:F.sub.2.sup.+ crystals are solved simultaneously and an efficient room temperature stable lasing of color centers is realized.